1. Field of the Invention
The present invention generally relates to an optical linear encoder, and, more particularly, to an optical linear encoder that is suitable for a clock signal generator employed in an information recording apparatus where a light beam intensity-modulated with information signals is deflected in an optical scanning system, and then a photo-sensitive recording medium is raster-scanned by the modulated light beam in the recording period.
2. Description of Prior Art
Various optical scanning types of information recording apparatus have been developed in which information is recorded under the control of clock signals produced by optically scanning an optical linear encoder.
As such an information recording apparatus, a computer output microfilm apparatus (referred to as "laser-COM" hereinafter) is known, in which print data (variable information) supplied from the computer and desirable form data (fixed information) are recorded on microforms by employing a laser beam as scanning light.
An optical scanning system of the above-described laser-COM will now be summarized with reference to FIG. 1.
An argon (Ar) laser 1 emits blue green light beams for recording purposes, which are indicated by "B". The blue green light beams B are intensity-modulated in an optical modulator 2 by video signals (will be discussed later) and thereafter pass through a first dichroic mirror 3. A helium-neon (He - Ne) laser 4 emits red light beams for reading purposes, which are denoted by "R". The red light beams R are incident upon a first reflecting mirror 5 and reflected therein and thereafter incident on the first dichroic mirror 3. The red light beams R are reflected on the first dichroic mirror 3 and mixed with the other light beams for recording purposes that have passed through this dichroic mirror 3. The combined light beams are incident on a rotating polyhedric mirror 7 through a second reflecting mirror 6. In this case, the first dichroic mirror 3 is designed to pass the blue and green light beams therethrough and to reflect the red light beams thereon.
The rotating polyhedric mirror 7 is rotated in a predetermined direction at a constant rate by a motor 9 to which a power is supplied from a motor drive circuit 8. As a result, the combined light beams R, B incident upon the respective mirror surface of the rotating polyhedric mirror 7 are reflected on these mirror surfaces and simultaneously deflected (referred to as "horizontal-deflected beams"). Then, the mixed light beams are converted into primary scanning light having a repetition period that is defined by the beam reflections occur in the respective mirror surfaces of the rotating polyhedric mirror 7. The primary scanning light is incident upon a second dichroic mirror 11 via a convergent optical system 10. The second dichroic mirror has such characteristics that the recording blue-green light beams and the reading red light beams can be transmitted there-through and a part of the reading red light beams can be reflected thereon. Accordingly, in the mixed light beams incident upon the second dichroic mirror 11, both the blue-green light beams B and the red light beams R pass toward a galvanometer 12, and the red light beams R are partially reflected and incident upon a linear encoder 13.
In response to saw-tooth driving signals supplied from a galvanometer driver 14, the galvanometer 12 deflects the recording light beams R, B in a direction substantially perpendicular to the horizontal deflecting direction (referred to as "vertical deflection"). As described above, the galvanometer driver 14 produces the saw-tooth driving signals based upon clock signals derived from a clock signal generator 15 (will be discussed later). For instance, counting these clock signals in a vertical address signal generator 16 in the vertical deflection period enables the vertical address signals to be produced. In response to these address signals, the galvanometer driver 14 produces the above-described saw-tooth driving signals.
Since the blue green light beams and also the red light beams vertically deflected by the galvanometer 12 have been converted into the one dimensional scanning light by the rotating polyhedric mirror 7, they become two dimensional scanning light by means of such vertical deflections. Then, the two dimensional scanning light is incident upon a third dichroic mirror 17, thereby splitting it into the blue green light and the red light.
The two dimensional scanning light of the blue green light beams passing through the third dichroic mirror 17 is focused on recording materials such as films via a focusing optical system 18 to raster-scan them. The other two dimensional scanning light of the red light beams split by the third dichroic mirror 17 is incident upon a form slide film 20A via a third reflecting mirror 19.
In a form slide film device 20, a plurality of form slide films 20A 20B, ---, 20N (where N represents the number of slides) are preset which are the most useable. Different slide images and writing frames constituted by a plurality of vertical and horizontal lines are recorded on these slide films 20A, 20B, --- , 20N. For the sake of simplicity, only two form slide films 20A and 20B are illustrated. One of these form slide films is selectively moved to a scanning position where it is scanned by the above two dimensional scanning light. As desired, the form slide films 20A, 20B, --- , 20N are arbitrarily detachable from the form slide device 20.
As seen from FIG. 1, the two dimensional scanning light R passes through the form slide film 20A and is converted in a first photomultiplier 21 to electric readout signals. The readout signals correspond to video signals of the writing frame image of the scanned form slide film 20A.
The red light beams R split by the second dichroic mirror 11 are, on the other hand, incident upon a linear encoder 13 to be one-dimensional-scanned. The linear encoder 13 is formed by a plurality of transparent and non-transparent line-shaped grids which are aligned parallel to the horizontal deflection direction and equidistantly separated to form a straight striped pattern. Pulsatory light obtained by scanning this linear encoder 13 by means of the horizontal deflection scanning light is converted by a second photomultiplier 22 into pulse signals as clock pulse signals. By applying these clock pulse signals to a phase-coupling type clock signal oscillator 23, clock signals are oscillated. The clock signals are used to synchronize the respective circuit elements of the laser-COM with each other under the desirable timings. The linear encoder 13, second photomultiplier 22, and clock signal oscillator 23 constitute a clock signal generating device 15.
Under the timing control of the clock signals derived from the clock signal generating device 15, character information corresponding to coded data from the character information source such as magnetic tapes etc. can be read out from a character generator 24 as video signals. These video signals derived from the character generator 24 are supplied to a signal composite circuit 25. While the form signals that are obtained by amplifying outputs of the first photomultiplier 21 in the amplifier 26 and thereafter shaping them in a level slicer 27 are supplied to the signal composite circuit 25, the above video signals are combined with the form signals in the signal composite circuit 25.
Thus the composite video signals are supplied through a modulator drive circuit 28 to the optical modulator 2 so as to intensity-modulate the recording light beams. As easily seen, the raster-scanned image projected toward the film F corresponds to an image formed due to the fact that the print data derived from the computer is written in a given position of the form frame selected by the form slide film.
As previously described in detail, the linear encoder 13 of the clock signal generating device 15 is constituted by the line-shaped grids equidistantly arranged as a striped pattern. Accordingly, if the deflection speed, or rate of the scanning light for this linear encoder 13 is constant, time periods of the pulsatory light incident upon the second photomultiplier 22 would become equal since the line-shaped grids are scanned by the scanning light beams having a constant deflection speed. In this case, the correct clock signals having no frequency variation can be produced from the clock signal generating device 15.
However, the scanning light incident upon the linear encoder 13 is traveled through a plurality of optical elements prior to being incident upon the linear encoder 13. As a result, it is considerably influenced by distortion of these optical elements, especially the focusing optical system (lens system) 10. When the rectangular image as shown in FIG. 2A is for instance transported through the optical system having such distortions, the transported image becomes optically distorted as shown in FIG. 2B. Thus, the scanning speed of the horizontal scanning light incident upon the linear encoder 13 via such an optical system is not constant, or varied, so that the linearity of the pulsatory light derived from the linear encoder 13 is hampered. The scanning light "L" intersecting the optical system having the distortion shown in FIG. 2B is conducted to the linear encoder 13. In this circumstance, the scanning speed of the scanning light beams, at the center a of the distorted image, incident upon the linear encoder 13 through the above-described distorted optical system is different from that of the scanning light beams at the ends b and c thereof. Accordingly the outputs of the linear encoder 13 contain the deflection distortion. The conventional linear encoder 13 shown in FIG. 3A in which the light transmission parts 29 and the light non-transmission parts 30 are equidistantly arranged delivers the pulsatory light of which occurence periods are not uniform. An interval between these light transmission parts 29 or non-transmission parts 30 is indicated by "T". The pulse signals derived from the second photomultiplier 22 to which such a pulsatory light is conducted contain non-uniform time periods such as signal intervals t.sub.1, t.sub.2, t.sub.3 and so on (see FIG. 3B). Consequently frequencies of the clock signals derived from the clock signal generating device 15 in synchronism with these pulse signals are changed in the deflection periods of the scanning light beams, so that as illustrated in FIG. 3C, the recorded image of the data (varied information) read out from the character generator 24 shown in FIG. 1 is considerably distorted on the film F.
An object of the present invention is to provide an optical linear encoder in which scanning light beams conducted into the clock signal generating device through the optical system having optical distortions are converted into a series of pulsatory light equivalent to the distortion-corrected scanning light so as to solve the conventional drawback occurring in the clock signal generating device.